Re: [Discuss-gnuradio] GMSK: GSM initial phase for a burst

[Jeffrey Karrels]

Great suggestion!

I have attached a couple of things:
1) The IQ plot (1e6 sample rate)
2) The burst data (MATLAB array)
3) The da vs time plot. (1 symbol = 3.69us)

Another quick question.  I can now see that after the amplitude ramp
up that there is a short section of negative phase movement (.7
radians clockwise) prior to the remaining counterclockwise rotations.
Should I expect a full (-pi/2) to start off the burst? Is my sampling
rate just not high enough to catch the remaining rotation? Any other
suggestions?

Thank you for your time
Jeff

On 6/18/07, Matt Ettus <address@hidden> wrote:
> Jeffrey Karrels wrote:
> > Hello everyone.
> >
> > I am looking at the frequency correction burst of a GSM signal.
> > According to spec the original data defines 148 bits of 0 that are
> > differentially encoded and GMSK modulated.  After the data goes
> > through the differential encoder it ends up as [-1,1,1, (142 1s),
> > 1,1,1].  I have taken this burst and plotted I vs Q (which is
> > attached).  I was expecting to see the amplitude ramp up followed by a
> > negative pi/2 phase shift, followed by 147 positive pi/2 phase shifts,
> > and a ramp down. I never see the initial negative phase shift though.
> > Am I missing something in my sanity? Any thoughts or ideas?
>
> Its hard to read the graph like that.  Plot angle vs time or even
> better, derivative of angle vs. time.
>
> Matt

fcch_data.zip
Description: Zip archive

fcch_iq.png
Description: PNG image

fcch_da.png
Description: PNG image